ready in 1 hr 55 min.
On the work surface, make a mound of the flour, icing sugar, lemon zest and a pinch of vanilla. Break the egg into the centre and dot the butter around the edge. Chop through all the ingredients with a large knife then work by hand to a firm dough. Take around 1/3 of the dough and knead in the cocoa. Wrap both types of dough in cling film and chill for at least 1 hour.
Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6 and line two baking trays with grease-proof paper.
On a floured surface, roll the dough out to 3-4 mm thick. Cut large stars out of the pale dough and small stars out of the dark dough, and place on the baking trays. Bake in the centre of the oven for around 10 minutes.
Carefully transfer the biscuits to a wire rack and leave to cool.
Stir the lemon juice and icing sugar together to a thick icing. Dab a little icing on the underside of the dark stars and stick them onto the pale stars.